Active Region Helicity

1. Active Region Helicity Exploring the Prominence/CME Connection Bruce W. Lites HMI Team Meeting 24 January 2005

2. Motivation • Magnetic helicity has hemispheric preference (filament geometry) – independent of solar cycle • Helicity must be removed from the Sun! • Generally conserved, especially on large scales • Would increase unbounded in each hemisphere • Prominences  CMEs are likely the means of ejecting this excess helicity • QUESTION: How is helicity injected into the solar atmosphere? • Surface motions acting on emerged fields rooted in photosphere? • Advection into atmosphere from pre-existing twist in interior?

3. Strong Signature of Hemispherical Preference for Right (Left) handed Geometry in North (South) Hemispheres. This corresponds to negative (positive) helicity in the north (south).

4. LASCO C2 Images Showing Eruption of Braided Prominence

6. Can we observe the twist emerging through the photosphere that ultimately ends up in the prominence fields? In the case of the quiet Sun, probably not.

7. ASP vector field measurements of a quiet region filament • Photospheric flux is confined to nearly vertical orientation because of buoyancy of the strong flux tubes. • Weak flux from Stokes V measurement exists throughout region, but it is too weak to be subjected to meaningful analysis.

8. Well then, how about measuring helicity emergence in active regions? • Some would argue that this has been demonstrated. • What fraction of global helicity emergence appears in active regions as opposed to the quiet Sun?

9. Average Force-Free Parameter  as Proxy for Magnetic Helicity (Pevtsov et al. 1995)

10. Zhang & Bao 1999 Current Helicity (not a conserved quantity) Force-Free Parameter  †Note: Stokes Q,U measured at line center: magneto-optical effects could cause serious, systematic errors in field azimuth.

11. Some recent analyses of ASP observations cast some doubt on the robustness of the active region helicity measurements: Ic Bapp (B)z

12. Now examine the force-free parameter z = (B)z/Bz |z| < 5 |z| < 1 • Conclusions: • Sunspot canopies give misleading results for z because fields are nearly horizontal (magnification of observational uncertainties) • Fine-scale structure is dominant  non-force free?

13. Note: • Plage fields are usually observed to be vertically oriented • Buoyancy and local convection dominate their vector orientation • Helicity migrates rapidly to upper layers • Thus, difficult to directly measure helicity emergence away from sunspots or pores after the initial emergence event

14. Where, Then, Do We Look for Emerging Twist of the Field? • This proposal: Examine the photospheric vector field under low-lying active region filaments: • If filaments (prominences) are the indicators of flux ropes emerged into the atmosphere, then they may show signatures of emerging helicity • Horizontal flux ropes will harbor current systems that are invisible to standard vector magnetogram analysis techniques, which are based solely on Jz • Active region filaments are fairly common

15. Pilot Study: AR Low-Lying AR Filaments Observed with the ASP • Limited data, but encouraging results • One observed filament showed persistent concave-upward field geometry indicating presence of flux rope in the atmosphere above • Tools being developed by van Ballegooijen to infer the flux rope current based on observed vector field, and filament geometry • HMI + AIA data may be able to provide a large statistical and quantitative data set for emerged and emerging helicity in active regions

16. History of active region shows persistent filament in plage region with continguous fields of opposite polarity.

17. Concave-upward field geometry at photosphere under PIL • Relative to sunspots, plage fields can be influenced by flux ropes in the atmosphere above • How can we quantify the flux rope properties?

18. Substantial horizontalfields under polarity inversion line (PIL) • H filament over PIL • Inverse polarity at PIL (fields point from negative towards positive polarity)  concave-upward field geometry at photosphere • PIL isolated from sunspots: only plage

19. Angle of horizontal field with respect to tangent to PIL: positive values indicate inverse polarity persists over much of length of the filament.

20. Empirical Models of CoronalMagnetic Flux Ropes – (van Ballegooijen 2004) • Challenge is to construct realistic, 3D magnetic models of solar active regions at one instant of time, regardless of how flux ropes are formed. • Models will be based on: • observed H-alpha filaments • observed coronal loop structures (e.g., TRACE, SDO) • observed photospheric vector fields (e.g., HMI) • Approximate as a nonlinear force free field (NLFFF) in chromosphere and corona.

21. Empirical Models – (van Ballegooijen 2004) New method for constructing NLFFFs containing flux ropes (van Ballegooijen, ApJ, 612, 519, 2004): Step 1: Extract magnetic field from MDI magnetogram SOHO/MDI 2004/06/19 1:00 UT

22. Empirical Models – (van Ballegooijen 2004) Step 2: select path along Polarity Inversion Line (PIL) at the location of an observed filament path filament from BBSO full-disk image

23. Empirical Models – (van Ballegooijen 2004) Step 3: Compute potential field (= overlying coronal arcade) Step 4: Create cavity above selected path Step 5: Insert flux rope into cavity (axial and poloidal fields) vertical cross-section of magnetic field near PIL potential field axial flux: poloidal flux:

24. Empirical Models – (van Ballegooijen 2004) Step 6: magneto-frictional relaxation with fixed vector potentials at the photosphere. Configuration after only 200 iterations:

25. Empirical Models – (van Ballegooijen 2004) Activity complex on April 8, 2000 with several filaments:

26. Empirical Models – (van Ballegooijen 2004) 3D magnetic model containing 4 flux ropes:

27. Empirical Models – (van Ballegooijen 2004) Close-up of filament #1:

28. Empirical Models – (van Ballegooijen 2004) Photospheric vector field below filament #1: Blue vectors: measurements, Advanced Stokes Polarimeter White vectors: 3D model. FOV: 40 arcsec

29. Simulated HMI Stokes profiles for observed ASP Stokes profiles at PIL: plage signal is present above noise

30. Conclusions • Diagnosis of helicity emergence is difficult based on observed photospheric vector field • Signatures of flux ropes in atmosphere abound: AIA will record structures such as filaments and twisted coronal arcades • HMI can provide photospheric vector fields as boundary conditions for modeling atmospheric flux ropes • SDO could provide a large statistical base of information on atmospheric flux ropes and their evolutionary history

33. First Concern: What is a Prominence? • Observationally, most CMEs associated with pre-existing prominence/filament (or filament channel) • One should first understand: • The prominence as a physical system, • Its relationship to the evolution of the photospheric field, • Its relationship to overlying coronal fields and structure. • Expectation: Only when armed with that information can one objectively tackle the problem of CME initiation

34. Essential Questions Surrounding the Prominence-CME Connection • What is (are) the typical prominence/coronal magnetic field configuration(s)? • How do these fields arise from emerging magnetic flux? • Do photospheric flows shape the field configuration, or • Are photospheric footpoint motions simply a signature of an emerging flux system? • Where does the cool, dense prominence mass come from? • Where does the prominence mass go? Focus of this presentation: Possible future avenues for answering these questions

35. Prominence models should embrace the following observed characteristics: From Hanle effect analyses (Leroy 1989): • Fields roughly aligned with underlying photospheric polarity inversion line • “Inverse” polarity in the majority of prominences (~75%) From physical reasoning: • Local “dips” in magnetic field support cool prominence material (otherwise it would rapidly drain out)

36. Sheared Magnetic Arcade Prominence Scenario • Features: • Photospheric shear confined to narrow strip near polarity inversion line • External large-scale unshearedbipolar field holds down the sheared flux • No net removal of flux or helicity from photosphere • Opposite polarities at photosphere separate continuously: no wholesale flux cancellation Aulanier, DeVore, Antiochos 2002, ApJ 567, L97

37. Flux Rope Models: Braided Field Topology is the Key Physical Characteristic

38. Empirical Models – Part 1 New method for constructing NLFFFs containing flux ropes (van Ballegooijen, ApJ, 612, 519, 2004): Step 1: Extract magnetic field from MDI magnetogram SOHO/MDI 2004/06/19 1:00 UT

39. Empirical Models Step 2: select path along Polarity Inversion Line (PIL) at the location of an observed filament path filament from BBSO full-disk image

40. Empirical Models Step 6: magneto-frictional relaxation with fixed vector potentials at the photosphere. Configuration after only 200 iterations:

41. Empirical Models Test convergence to NLFFF:

42. Empirical Models After 5000 iterations, the flux rope is roughly in equilibrium with its surroundings: Helical field lines at the edge: Untwisted field in the core:

43. Empirical Models What the flux rope would look like near the solar limb:

44. Empirical Models Predicted photospheric vector field is aligned with the PIL: FOV: 1 arcmin Bmax: 600 G

45. Empirical Models Comparison with filament and coronal loop observations: BBSO H-alpha TRACE 171 Å

46. Empirical Models – (van Ballegooijen 2004) Vertical currents (grey) and magnetic field at height 5.8 Mm:

47. Summary – (van Ballegooijen 2004) • Coronal flux ropes can be formed by reconnection associated with photospheric flux cancellation. • Observations indicate the cores of flux ropes are relatively untwisted. • Can be explained with theoretical models that include coronal magnetic diffusion. • Need to construct empirical models based on actual observed photospheric vector fields, filaments, and coronal loops.

49. Coronal Field Structure as Exemplified by Two Prominence Scenarios Sheared Arcade Model Flux Rope Models

50. LASCO C2 Images Showing Eruption of Braided Prominence